Use (void) for no parameters, not ()

This ensures we build with -Werror=missing-parameter-type.

https://bugzilla.gnome.org/show_bug.cgi?id=687385

gobject/tests/threadtests.c

@@ -41,7 +41,7 @@ call_counter_init (gpointer tclass)
     }
 }
 
-static void interface_per_class_init () { call_counter_init (NULL); }
+static void interface_per_class_init (void) { call_counter_init (NULL); }
 
 /* define 3 test interfaces */
 typedef GTypeInterface MyFace0Interface;